This might give you pause before heating up leftovers for lunch at work: among the dirtiest spots in a typical office are the microwave door and refrigerator door handles in the break room. You probably already touched both today.

The swabs were analyzed with an ATP meter, a device commonly used to assess sanitary conditions in industry. It measures levels of adenosine triphosphate (ATP), a molecule found in all animal, vegetable, bacteria, yeast and mold cells. High ATP levels are present in food or other organic residues left on surfaces. The more ATP found on a surface, the more likely it’s flourishing with bacteria and viruses.

An ATP reading of over 100 suggests a surface could use a scrub-down. Readings of 300 or higher are considered officially dirty and at high risk for spreading illness. Note the meter doesn’t directly measure germs, but the dirty surfaces they cling to.

The dirtiest office surfaces found to have ATP counts of 300 or higher were as follows:

“A lot of people are aware of the risk of germs in the restroom, but areas like break rooms have not received the same degree of attention,” study consultant Dr. Charles Gerba, professor of microbiology at the University of Arizona, said in a statement. “This study demonstrates that contamination can be spread throughout the workplace when office workers heat up lunch, make coffee or simply type on their keyboards.”

It’s impossible to avoid germs entirely, but according to Brad Reynolds of Kimberly-Clark Professional’s Healthy Workplace Project, diligent washing, wiping and sanitizing can help office workers reduce their rates of cold, flu and stomach illness by up to 80%.

Here are some of Gerba’s tips for protecting yourself from catching your co-workers’ coughs, sniffles or other germs:

Keep hand sanitizer at your desk and use it immediately after every meeting or conference

Wash your hands or use hand sanitizer when you get to work, especially after riding mass transit, such as trains or buses

Use disinfectant wipes to clean your desk at least once a day, particularly if you eat at your desk

Use disinfectant wipes to sanitize high-touch areas in a break room, or use paper towels to touch them